Performance requested of image-forming systems and toner has started to become additionally sophisticated in recent years in association with widespread use of image-forming apparatuses such as a copying machine and a printer. To be specific, an improvement in quality of an image formed with any such system or toner and an increase in speed at which the image is formed have been requested, and furthermore, the kinds of transfer materials to be used have been covering a broad spectrum: a transfer material such as coat paper as well as plain paper has started to be used.
Of the image-forming systems, a one-component developing system is preferably used as a developing system because a developing device to be used is of a simple structure, causes a small amount of trouble, has a long lifetime, and can be easily maintained.
Several approaches have been known about the one-component developing system, and a jumping developing method is one of them. The jumping developing method is a method involving: causing toner charged by triboelectric charging to fly onto a photosensitive member with a developing bias; and visualizing an electrostatic image on the photosensitive member as a toner image.
In this case, the toner, which has a proper charge quantity, follows the developing bias to reciprocate between the photosensitive member and a developing sleeve. As a result, the toner image is formed in an image portion, and the toner flying toward a non-image portion returns to the developing sleeve, whereby a clear image is obtained.
In addition, the following approach has been employed as one transfer step: a voltage opposite in polarity to that of the toner is applied to a transfer material, and the toner image on the photosensitive member is caused to fly toward the transfer material by a Coulomb force between the toner and the transfer material.
In order that the improvement in image quality and the increase in speed requested in the market may be achieved in such image-forming system as described above, a photosensitive member having the following characteristics is needed: a clear electrostatic image can be formed on the photosensitive member, and a desired electric field can be formed between the photosensitive member and the developing sleeve, and between the photosensitive member and the transfer material in a developing zone and a transfer zone.
Further, a toner having good charging stability is needed in order that the electrostatic image formed on the photosensitive member may be uniformly compensated.
When a photosensitive member on which an electrostatic image cannot be clearly formed and a toner having low charging stability are used, an electric field to be formed in the developing zone or transfer zone cannot be the desired electric field, and furthermore, the electric potential of the toner image on the photosensitive member is apt to be nonuniform. As a result, an image defect such as scattering or tailing occurs.
For example, a magnetic iron oxide particle is one factor for determining the charge quantity and charging stability of toner. The magnetic iron oxide particle exposed to the surface of the toner is expected to serve as a leak point for charging. In particular, it has been known that FeO in the magnetic iron oxide particle has a function of reducing the resistance of the magnetic iron oxide particle. It has also been known that the content of FeO in the magnetic iron oxide particle largely contributes to the charging stability of the toner.
For example, Patent Document 1 proposes a magnetic iron oxide particle having the following characteristic: an FeO amount in a layer from the surface of the particle to a thickness corresponding to 3.5% of the radius of the particle is specified to a low value so that charge leak may be suppressed, and a saturation time for the triboelectric charging of a toner containing the particle may be shortened. However, the charging stability of the toner may be insufficient because the FeO amount is small.
In addition, Patent Document 2 proposes a magnetic iron oxide particle that imparts good charging stability to toner irrespective of an environment under which the toner is used by the following procedure: a surface FeO amount is specified for three stages of an iron element dissolution ratio, i.e., 5%, 10%, and 15% so that an FeO amount may be specified to a large value.
In addition, with regard to a photosensitive member, a photosensitive member using hydrogenated amorphous silicon carbide (hereinafter referred to as “a-SiC:H”) in its surface layer is preferably used in a high-speedmachine requested to show high durable stability and high reliability. The photosensitive member using a-SiC:H in its surface layer has the following advantages: the photosensitive member has a high surface hardness, and is excellent in durability and heat resistance. Accordingly, the photosensitive member shows nearly no deterioration due to repeated use, so the photosensitive member has been expected to provide the following merit: clear electrostatic images can be formed on the photosensitive member over a long time period.
However, the a-SiC:H surface layer involves, for example, the following problem: a corona product typified by NOX or SOX adheres to the surface layer so as to be one cause for the collapse of an electrostatic image. When the corona product adheres to the surface of the photosensitive member, the corona product captures moisture in the air, reduces the resistance of the surface, and causes the deletion of the electrostatic image. A general measure against the foregoing is, for example, a method involving attaching a heater to the photosensitive member to reduce the amount of moisture adhering to the surface. In addition, Patent Document 3 proposes a photosensitive member having the following characteristic: a hydrophobic fluorine atom is incorporated into the surface protective layer of the photosensitive member so that the reactivity of the surface protective layer with a corona product or water may be reduced.
In addition, the a-SiC:H surface layer has a large number of dangling bonds, and the dangling bonds are known to capture photo carriers to inhibit the formation of a clear electrostatic image. In view of the foregoing, Patent Document 4 proposes a method of providing a photosensitive member having a small number of dangling bonds, the method having the following characteristics: the method is a method of forming a photosensitive member by plasma CVD in which a plasma density is specified, and a photosensitive member layer is formed by the method.
As described above, a magnetic iron oxide particle having good charging stability and a photosensitive member on which a clear electrostatic image can be formed have been proposed.
Meanwhile, a transfer material having a smooth surface such as coat paper has started to be used in the market in association with the requests for the increase in speed and the improvement in image quality. In the coat paper, an image printed on the paper is faithfully reproduced, and the quality of the image is improved; for example, the gloss of the image is improved, and the non-uniformity of the gloss is reduced.
However, an image defect such as minute scattering or tailing which is inconspicuous and hence not perceived as a problem on plain paper tends to appear remarkably in the coat paper having a smooth surface.
A possible reason for the foregoing is as described below. The surface of the plain paper has unevenness due to its fibers. Accordingly, even when minute scattering or tailing occurs, the minute scattering or tailing is embedded in a gap between the fibers so as to be of no concern when viewed with the eyes. In the case of the coat paper having a smooth surface, however, even minute scattering remains on the surface so as to be conspicuous.
An additional improvement for obviating an image defect such as minute scattering or tailing is needed in order that image formation may be adapted to a wide variety of transfer materials in the recent trends toward the increase in speed and the improvement in image quality.    Patent Citation 1: JP 2001-2426 A    Patent Citation 2: JP 04-338971 A    Patent Citation 3: JP 2002-207305 A    Patent Citation 4: JP 08-211641 A